Gargron
Mastodon includes the omniauth gem, through which e.g. CAS and SAML systems are connected. Using omniauth it should be possible to support sign up through any OAuth service.
However, I don’t see how I could make this any easier to integrate since every service requires some level of customization. It might be as easy as adding a new line in here for some: https://github.com/tootsuite/mastodon/blob/master/config/initializers/omniauth.rb I haven’t tried.